A light source generally known to have a high efficiency is the low pressure sodium lamp. It has the highest luminous efficiency among artificial light sources which are commonly used, and attains a total efficiency of 1091 m/W with a lamp having output power of 55 W. However, most of emission form the low pressure sodium lamp is concentrated in an emission line of 589 nm called D-line, namely monochromatic light of orange-yellowish color, which disables it to distinguish different colors.
Meanwhile, a three band radiation type fluorescent lamp has been developed as a high-efficiency light source capable of good reproduction of colors by concentrating the light in the visible radiation band into red (R), green (G) and blue (B).
Simulation has been conducted to verify the possibility of a light source which has a luminous efficiency higher than that of a light source which has an emission spectrum of three band radiation type, and has some degree of color rendering properties, by means of radiation spectrum of dual band radiation type where light of visible radiation band is concentrated into two wavelength bands.
Simulation for optimization of light source of dual band radiation type of the prior art is reported in literature such as H. D. Einhorn and F. D. Einhorn "Inherent Efficiency and Colour Rendering of White Light Sources", Illuminating Engineering, P154, March 1967 and H. F. Ivey "Color and Efficiency of Fluorescent and Fluorescent-Mercury Lamps", Journal of the Optical Society of America, Vol. 62, No. 6, P814, 1972.
These works conducted numerical simulation of dual-wavelength optimization based on the approximation of spectral distribution of phosphor by the Gaussian distribution and on such a concept of color rendering properties as evaluating, in terms of color difference, the general color rendering index Ra of the prior art, namely the color shift between the appearance of color of a color chip illuminated by a reference light source and the appearance of color of a color chip illuminated by a light source.
As a result, mixture of blue spectrum near 450 nm and yellow spectrum near 580 nm has been considered to be a white light source that shows the highest efficiency with respect to the optimization of dual band radiation type in terms of simulation. (Although a yellow and blue light source of dual band radiation type has been considered to be most desirable in the prior art from the view point of efficiency and Ra, now the yellow and blue light source of dual band radiation type described above is redefined as Y-B base light source of dual band radiation type from the view point of opponent-color response of human color vision, and a R-G base light source of dual band radiation type is newly defined based on a system of another opponent-color response.
However, when the efficiencies of phosphors in practical use are investigated, there is no phosphor having high efficiency among phosphors that emit at peak wavelengths corresponding to yellow and blue. In case of a phosphor of radiation spectrum approximated by the Gaussian distribution with quantum efficiency assumed to be constant over the entire wavelength range, simulation may indicate a high theoretical efficiency for the Y-B base light source of dual band radiation type but the emission from the actual phosphor or light source sometimes shows varying quantum efficiency or includes sub-wavelength of emission. Thus it is difficult to achieve the highest efficiency as predicted by simulation, and such a Y-B base light source of dual band radiation type has not been put in practical use.
Highest luminous efficiency can be achieved by concentrating the light in visible radiation band at one wavelength as in the case of the low pressure sodium lamp. In this case, however, because the radiation spectrum is a single line spectrum, color discrimination is impossible when illuminated with such a light source. Therefore, lamps having a very high efficiency but poor color rendering properties such as the low pressure sodium lamp are used in illumination of roads and road tunnels where emphasis is placed on efficiency.
While colors have the role of providing various information on the visual environment for humans, colors have a particularly great role among the visual information received by drivers of cars running on roads or in tunnels. For example, distinguishing lane marking white and yellow in a tunnel is very important to know whether lane changing is permitted or not. However, on roads and in tunnels which are illuminated by the low pressure sodium lamps, it has been difficult to distinguish the white and yellow lane markings on the road surface.
While colors used on traffic signs include red, yellow, green, blue, white and black on roads and in tunnels which are illuminated by the low pressure sodium lamps, it has been difficult to distinguish different colors of these signs (therefore traffic signs with built-in lamps are used in tunnels).
The important thing is that, the red of surface color be recognized red. Because red, in particular, which is coded for important meanings such as danger, prohibition, stop and fire fighting. Therefore important point in improving the visual environment from the view point of safety.
In case the Y-B base light source of dual band radiation type described previously is introduced in such a situation, there occurs such a problem that the probability of recognizing red which is an important color for the indication of danger is reduced due to the lack of spectrum at wavelengths 600 nm and longer.
In order to solve the problems of the prior art described above, the present invention has an objective of providing a practical light source which, when applied to road illumination or tunnel illumination, enables it to distinguish yellow and white road surface markings and to recognize the colors on road signs (particularly red) while maintaining a high efficiency, and enables color recognition at minimum necessary level in other applications, while maintaining a high efficiency.
To sum up, the invention intends to realize a light source that achieves a high efficiency while ensuring categorical recognition of colors at a minimum required level, while it is tried to improve the color rendering properties with a major objective aimed at exactly reproducing colors as represented by the general color rendering index Ra in the development of an illumination light source of the prior art.